Rheological engineering of perovskite suspension toward high-resolution X-ray flat-panel detector

Solution-processed polycrystalline perovskite film is promising for the next generation X-ray imaging. However, the spatial resolution of current perovskite X-ray panel detectors is far lower than the theoretical limit. Herein we find that the pixel level non-uniformity, also known as fixed pattern noise, is the chief culprit affecting the signal-to-noise ratio and reducing the resolution of perovskite detectors. We report a synergistic strategy of rheological engineering the perovskite suspensions to achieve X-ray flat panel detectors with pixel-level high uniformity and near-to-limit spatial resolution. Our approach includes the addition of methylammonium iodide and polyacrylonitrile to the perovskite suspension, to synergistically enhance the flowability and particle stability of the oversaturated solution. The obtained suspension perfectly suits for the blade-coating process, avoiding the uneven distribution of solutes and particles within perovskite films. The assembled perovskite panel detector exhibits greatly improved fixed pattern noise value (1.39%), high sensitivity (2.24 × 104 μC Gyair−1 cm−2), low detection limit (28.57 nGyair·s−1) as well as good working stability, close to the performance of single crystal detectors. Moreover, the detector achieves a near-to-limit resolution of 0.51 lp/pix.


Point-to-point response
Reviewer #1: The authors show a result based paper with outstanding performance of their perovskite based flat panel detectors.Most of the measurements are shown in a good and clear way, but i have some questions/remarks: 1. Figure 2 c and d: Can you provide the Measurement data behind the normalized results?The curves c and d show exactly the same behavior -i would like to know more about the measurements.
Are their statistical data which can be provided or is it just one measurement of one sample?How was the data normalized and why?
Response: Thanks for the reviewer's recognition of our work.We provide the original measurement results and the statistical data in the new version.The measurement data for Figure 2c and d is shown in Figure R1.For Figure 2c and d, we normalized the results by dividing the current density at pixel size of 100 μm pixel, to better illustrate the variation trend.For the original data (Figure R1), the different magnitudes of the current density make the comparison difficult.Anyway, we could see that the dark current density for blading-grade suspension is much lower than the pristine suspension, which is due to the suppressed trap density by PAN in the suspension (as illustrated in Figure 3).We also measured the statistical results (5 samples for each group).The films were prepared on the same pixelated substrate described in the manuscript.The results show similar trend that pristine suspension with a large variation under different pixel sizes.We have accordingly revised the manuscript at Page 10, Line 11 to 14, and Supplementary Information at Page 12 to 13.

Did you calculate the electron hole pair creation energy? Could you add it here in the paper?
This would be nice.
We further tried to calculate the actual EHP creation energy with the method reported by Sarah Deumel (Nat. Electron. 2021, 4, 681-688), In which, the function Aq(E) was the percentage of absorbed photons (Figure R3b), which was calculated from the total attenuation coefficient α for MAPbI3 taken from the NIST XCOM cross- section database (Figure R3a).The function X(E) was the simulated X-ray spectrum with a RQA3 filtration (Figure R3b).
We observed that the actual EHP creation energy was lower than the theoretical value of 4.53 eV (Figure R3c).We attributed this to the photoconductive gain effect, which resulted from the photoexcited electrons trapped by defects.This enabled more holes to travel between the electrodes multiple times before recombination.As a result, many photoexcited electrons were counted repeatedly in the calculation process.We have revised the manuscript at Page13, Line 13 to 14 and Supplementary Information at Page 5.

Did you investigate a gain in your X-ray measurements?
Response: The gain can be calculated with the method documented in our previous work (Nat.Photon. 2022, 16, 575-581, Nat. Commun. 2023, 14, 626, J. Phys. Chem. Lett. 2021, 12, 6961-6966.).Photoconductive gain is caused by defects in materials that can trap electrons (or holes).These trapped charges allow opposite charges like holes (or electrons) to move between the electrodes repeatedly, boosting the signal.As the light intensity decreases, the proportion of trapped charge carriers will increase and lead to nonlinear behavior especially at low dose rate.

Photonics
We noticed that the SNR did not change linearly with dose rate in many published articles because of photoconductive gain.Figure R3a exhibits a representative result in a recent work (Nat Photon. 2022, 16, 575-581.).SNR would be >0 when the dose rate is 0 Gy s -1 if the data were fitted linearly.
In our work, we experimentally monitored the dose rate until SNR<3, and we obtained the detection limit value with SNR=3.The film from blading-grade suspension exhibited the detection limit of 27.9 nGyair s -1 , and that from pristine suspension is 106.9 nGyair s -1 (Figure R4b).We have revised the manuscript at Page 2 Line13, Page 13 Line 17-22 and Page 18 Line 3.

Could you explain why you used RQA3? For radiography, RQA5 is usually used.
Response: Thanks for the reviewer's suggestion.RQA3, 5, 7 were set for different purposes.Both RQA3 and RQA5 are used for general radiography (the IEC standard 62220-1).RQA3 is suitable in neonatal, pediatric extremities imaging and mammography, while RQA5 is commonly used to image extremities, head and shoulder in adults (RSNA 2003, 27710, 37-47.).We have added the description of the potential application field at Page 17 Line 1-3.
Reviewer #2: This manuscript realized the improvement of X-ray detection properties of bladecoated perovskite thick film-based X-ray detectors through rheological engineering of perovskite solution.As the blade-coating process has great potential in terms of its scalability, this strategy is very useful.On the other hand, regarding the figure-of-merit, such as the sensitivity and spatial resolution, this paper is missing proper characterization.This reviewer will suggest major revisions.
Here are the comments: (1) In the introduction part, "In view of this, many researchers are committed to the utilization of perovskite polycrystalline thick films in X-ray FPDs", the authors should cite several papers which summarized most of the perovskite-based X-ray FPDs.Chem.Mater. 2022, 34, 12, 5323-5333 Response: Thanks for the reviewer's appreciation.We have added the suggested reference and also updated the refs in other parts.
(2) Figure 1b is unacceptable.The lines/curves are from just two points or even just one point.
Besides, the definition of stability is unclear.The reviewer suggests removing the light-blue background in Figure 1.
Response: Thanks for the good point.We have measured the viscosity of precursors with different MAI addition (15, 20, 30, 45 and 60 mg/ml) and the sedimentation time of these suspension before and after the addition of PAN.We also replace the term of stability by the sedimentation time.The destabilization time was defined as the time required to form 1-mm thick supernatant after stopping the stirring of the suspension, while longer time represents more stable suspension.We have revised  (3) The intent of Figures 2c and d is unclear.
Response: In Figure 2c and d, we normalized the results by dividing the current density at pixel size of 100 μm pixel, to better illustrate the variation trend.For the original data (Figure R1), the different magnitudes of the current density make the comparison difficult.Figure 2c and d are used to show the variation of current density on pixel size, since we notice that the poor homogeneity during film fabrication would seriously affect the pixel contact, as illustrated in Figure 2b.The blading-grade suspension could effectively reduce the inhomogeneity at small pixels.Moreover, the smaller pixels would be affected by the contact effect more seriously.
We have added the measurement data and statistical results in the revised version, as shown in above (Reviewer 1, Question 1).For the measurement data, we could see that the dark current density for blading-grade suspension is much lower than the pristine suspension, which is due to the suppressed trap density by PAN in the suspension.We also measured the statistical results (5 samples for each group).The films were prepared on the same pixelated substrate described in the manuscript.The results show similar trend that pristine suspension with a large variation under different pixel sizes.
We have accordingly revised the manuscript at Page 10, Line 11 to 14 and Supplementary Information at Page 12 to 13.
(4) Please clarify the photon source of the photocurrent for μτ product calculation (Figure 3b).The reviewer supposes that it is X-ray photons as shown in Figure 3c.But in this case, the μτ products can be overestimated as the carrier transport distance for a part of generated carriers will be shorter than the thickness.
From the lifetime (958.86 ns) and mobility-lifetime product (5.31x10-3cm2/V) in the manuscript, the mobility is calculated to be 5,540 cm2/V/s.Even though this material is a polycrystalline film, this mobility is much higher than that of the single crystal of MAPbI3 (10-1000 cm2/V/s).

Response:
The photon source of the photocurrent for μτ product calculation is UV light (365 nm, NO.M365L, Zolix).The estimated absorption depth is less than 320 nm, much lower than the film thickness (300 μm).Then we thought the derived μτ product was not overestimated.
To answer the second question, we need to clarify the meaning of the μτ product.The lifetime τ in μτ is different from the photoluminescence lifetime.Our photoconductive device exhibited photoconductive gain effect.Photoconductive gain is caused by defects in materials that can trap electrons (or holes).These trapped charges allow opposite charges like holes (or electrons) with longer lifetime and moving between the electrodes repeatedly.Thus, the lifetime in μτ product is the enhanced lifetime of the opposite carriers without trapping (in most case, majority carriers).Rev. 2023, 10, 011406) Thereby, we can not simply calculate μ by dividing μτ with photoluminescence lifetime τ.In the following table, we also summarized the reported μτ product and photoluminescence lifetime values.
If we divide μτ with photoluminescence lifetime, the calculated mobility value is also unreasonable.
The manuscript needs to clarify the electric field for Figure 3g (S/N ratio) and Figure 3h (stability).
As perovskite films usually suffer from ion migration problems, the signal stability should be described with the electric field.(6) The unit of resolution that is important in this manuscript is "lp/pixel".The authors need to justify using "lp/pixel" not "lp/mm".As the FPD in this manuscript is using 150 µm pixel pitch, which is larger than that of previous reports (70 µm or 50 µm), achieving near the theoretical limit of lp/pixel should be easier compared to the previous reports.The reviewer couldn't believe that this paper made a significant improvement in terms of spatial resolution.

Response
If the authors have achieved a higher resolution than in previous reports, please report images of a detailed object rather than the screwdriver.

Response:
The resolution of detectors depends on pixel size and other factors like the uniformity of film.The influence of pixel size follows the equation below, We used "lp/pixel" instead of "lp/mm", because we wanted to eliminate the influence of pixel size on the numbers that represented resolution, when we were discussing how the uniformity of film affected the resolution of detector.The theoretical maximum resolution for previous reports and our work should be 16.5 lp/mm for 50 μm pixel, 11.8 lp/mm for 70 μm pixel and 5.5 lp/mm for 150 μm pixel (our work).However, the obtained resolution was only 3.3 lp/mm for 50 μm pixel (Nature Electronics, 2021, 4, 681-688) and 3.1 lp/mm for 70 μm pixel (Nature, 2017, 550, 87-91), much lower than the theoretical value.
In our work, we could obtain the resolution of 3.4 lp/mm, although the pixel size (150 μm) is much larger than the two previous works.Additionally, we obtained X-ray imaging result of the line pair card, as shown below.The lines at 3.4 lp/mm can be clearly recognized, supporting the claim of high resolution.

REVIEWER COMMENTS
Reviewer #2 (Remarks to the Author): The manuscript has been substantially revised.I think it's great.I am Reviewer 2, and I have some comments, including a reply to Reviewer 1.
Reviewer 1's Q1 and Reviewer 2's Q3: The data is organized and shown, and I think it is an accurate response.At least enough to demonstrate the usefulness of PAN.
The reason is stated as "which is due to the suppressed trap density by PAN in the suspension".Why does trap density in pristine suspension decrease as pixel size increases?
This is not my intention to deny the usefulness of PAN, but my intention is that if the problem or reason for films obtained by "pristine suspension" is understood, it may lead to other solutions.
Reviewer 1's Q4: Not limited to perovskite materials, a phenomenon in which optical properties (response, lifetime, etc.) change nonlinearly with light intensity is observed.Therefore, we agree that nonlinear effects may be present in this material as well.
Therefore, it is recommended to show the fitted line in the range where linearity is maintained, rather than linearly fitting up to the Dose rate=0 point, for example, inset in Figure R4.
Reviewer 2's Q4: We don't see the red curve in Supplementary Note 3 (Figure S3-c).The authors may are referring to Figure R3 in the response.
The light intensity dependence is described above.Therefore, it is a bit questionable to simply describe the lifetime measured at one light intensity.However, I think it is sufficient here if at least the light intensity is clearly indicated as an experimental condition and the result of benefiting from the photoconductive gain.
Also, I think that Figure 3 I generally agree with this statement, but I do not agree with determining the superiority or inferiority of the resolution based on the difference (percentage) from the theoretical limit.
The authors claim high resolution by achieving 3.4 lp/mm even under the condition that the theoretical limit is 5.5 lp/mm (150 μm).However, both results of references and this work have almost the same resolution, around 3.3 lp/mm.It is unlikely that 150 μm pixels would be difficult to manufacture, so the methods in the two papers being compared ((Nature Electronics, 2021, 4, 681-688) and(Nature, 2017, 550, 87-91)) are expected to yield similar values for 150 μm pixels.So it's not a fair comparison.
Reviewer #3 (Remarks to the Author): The authors report on a new way to prepare and stabilize a perovskite suspension for manufacturing perovskite-based x-ray detectors.By adding polyacrylonitrile (PAN) to the suspension the viscosity of the solution could be kept at a low level by simultaneously increasing the sedimentation time greatly improving the produced MAPbI3 layer quality and its x-ray detecting properties.
The paper presents clear and well-supported results, with sufficient data and explanations to support its conclusions.Additionally, it reports significant improvements in the performance of MAPbI3-based x-ray detectors, surpassing previously reported results.All comments from the previous reviewers have been adequately addressed and incorporated into the revised manuscript.
While the revised manuscript is overall well-written and presents clear results, there are a few issues that could be addressed to further improve the paper.
1. To accurately compare the performance of the x-ray detector reported in this paper with the results from references 4 and 6, it would be helpful to include measurements using a higher energy x-ray spectrum.Reference 4 is using a standard RQA5 spectrum while reference 6 is using an even higher energy spectrum of 100kV filtered with 3mm Al.While it is true that RQA3 is suitable in neonatal, pediatric extremities imaging and mammography, comparative measurements should be done with the same or at least a similar spectrum.Here RQA5 is the de facto standard in the industry.Furthermore, it would be helpful to add DQE measurements to add a figure of merit which can be easily compared with commercially available detectors and clearly shows the image formation capabilities of the x-ray detector.
2. While providing supplementary information to support your measurements and claims is appreciated, it is best to avoid directly referencing this information within the manuscript itself (e.g.page 7 line 8).The manuscript should be self-contained, with all the necessary information to understand its ideas and results included within the text, without the need to refer to supplementary information.

Figure R1 .
Figure R1.(a) The dark current densities for different pixel sizes.(b) The X-ray response current densities for different pixel sizes.

Figure R2 .
Figure R2.(a) Statistical data of the dark current and X-ray response for films from pristine suspension.(b) Statistical data of the dark current and X-ray response for films from blading-grade suspension.

Figure R3 .
Figure R3.(a) The attenuation coefficient of MAPbI3.(b) The simulated X-ray spectrum with a RQA 3 filtration (black line) and the percentage of absorbed photons vs photon energy (red).(c) EHP creation energy (black) and gain of film (red) from blading-grade suspension.

Figure R4 .
Figure R4.(a) The SNR versus dose rate in Nat.Photon.2022, 16, 575-581.(b) The SNR versus dose rate figure of our devices with x and y axis in log style.

Figure R5 .
Figure R5.The viscosity and destabilization time of MAPbI3 suspension with excessive of MAI and PAN.
photoluminescence is caused by the recombination of minority and majority carriers and the minority determines the overall rate (ACS Appl.Mater.Interfaces 2013, 5(20), 10302-10309).In one recent work, Lin et.al. found that the lifetime of majority carriers (10 to 100 μs) is totally different from lifetime (~1 μs) measured in time-related photoluminescence spectra.(Appli.Phys.
have revised the manuscript at Page 12 Line 10 to 22, Page 20 Line 21 to 22 and Page 21 Line 1.

:
photocurrent of blading-grade film changed from 42.08 nA to 41.61 nA.We have revised the manuscript at Page 2 Line 13, Page13 Line 10-11, Page 14 Line 2-6 and Page 18 Line 3.

Figure R6 .
Figure R6.(a) The time dependent current density curve of the film from blading-grade suspension

Figure R7 .
Figure R7.(a) The x-ray imaging of a line pair card.(b) The MTF curve in lp/mm for blading grade suspension.
-b should be shown by a linear vertical axis.If possible, clearly indicate the difference between fitting and experimental results.Reviewer 2's Q6:

Table R1 .
The documented μτ product, photoluminescence lifetime, and derived mobility value.